Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US ______.

A．A report of theft of union funds by company officials.
B．An article about a little girl returning a large amount of money she founded in the street.
C．A scholarly analysis of all economic issue of national importance.
D．A story about land being given away in the West,

A．explain why so many different periodicals were published
B．prove that weekly periodicals were more successful than daily papers
C．show the difference between reading habits before and after the Civil War
D．support the belief that Americans were enthusiastic readers of periodicals